The present invention relates to a method for the continuous heat treatment and packaging of a liquid product, in particular with the object of reducing the content of micro-organisms of the product, the product being heated for a short time together with, and surrounded by, the packing material in which it is to be enclosed and subsequently cooled again. The invention also relates to an arrangement for the carrying out of the method.
In the technique of packaging, products to be packaged are subjected to a heat treatment before the actual packaging, mostly in order to reduce the content of bacteria and other micro-organisms in the product. It is of course primarily foodstuffs which are subjected to such a heat treatment, the ultimate object of the heat treatment being in general the prolongation of the keeping quality of the product. If the heat treatment is carried out long enough to prevent all bacterial growth, it is said, in general, that the product has been sterilized, which means that it can be kept in its package for a very long time (a number of months) without the product turning sour or being impaired in some other manner through microbiological effects. It is a pre-condition, however, if the product is to remain durable in its package, that the packaging of the product has to be carried out under aseptic conditions and that the parts of the packing material which come into contact with the sterilized contents are aseptic.
It is known that milk, fruit juices, water etc. can be sterilized by means of heat treatment and subsequent packaging of the sterilized product under aseptic conditions into previously sterilized packing material, whereupon the packed product can be kept in its unopened package for a very long time. This packing process, in principle, is divided into two stages, namely:
(a) heat treatment of the product with the object of reducing the content of micro-organisms, and
(b) packaging of the product comprising treatment of the packing material in order to sterilize the surfaces which come into contact with the product.
The heat treatment of the product which for example may be milk, can be carried out in any known apparatus, e.g. a plate apparatus which, in principle, is a heat exchanger where the heat-emitting medium flows along one path of flow and the milk intended for heat treatment flows along an opposite path of flow, the two flow media being separated by thin metal walls which readily transfer thermal energy from the heat-emitting medium to the heat-absorbing medium. Such devices termed "plate apparatuses", are used mainly for pasteurizing milk, that is to say heating to approx. 90.degree. C. in order to neutralize pathogenic bacteria. If a complete sterilization of the milk is to be carried out, a more comprehensive heat treatment and a heating to 140.degree. C. for a few seconds is required. Such a heating, as a rule, is not carried out in the so-called plate apparatuses, but in other types of heating arrangements where usually a jet or a film of milk is made to encounter a flow of superheated steam, the milk being heated rapidly to sterilization temperature. The heat-treated milk can be collected in sterilized tanks or containers awaiting packaging.
For the actual packaging, the sterilized milk is conducted under aseptic conditions to a packing machine wherein a web or a blank of packing material is sterilized internally before the sterilized milk is introduced. This filling and closing process must take place in a sterile room so as to hinder the sterilized milk or the sterilized packing material from being infected by bacteria present in the air.
The most customary and most rational method is to start off with a packing material web with a plastic-coated inside, this packing material web, possibly after treatment with a liquid sterilizing agent, e.g. hydrogen peroxide, being formed to a tube in that longitudinal edges of the web are combined with one another in a tight joint, whereupon the contents are introduced into the internally sterilized tube which, by means of repeated transverse seals, perpendicularly to the tube axis, is divided to form individual package units which can be separated by cutting through the sealing zones. In certain cases the sterilizing effect of the packing material is intensified by allowing the tube of packing material formed in the above-described manner to pass a source of heat which, by means of radiant heat or in some other manner, heats the plastic inside of the tube to such an extent that any micro-organisms present on the packing material web are rendered harmless at the same time as any residues of sterilizing agent are evaporated.
Another method for the manufacture of aseptic packages consists in first making blanks which, with the help of automatic machines, can be transformed to boxes provided with a base. These boxes can subsequently be sterilized on the inside in that hydrogen peroxide is introduced into the packing box in vapor form or in the form of small liquid particles, whereupon hot air or steam is blown into the packing box, on the one hand to enhance the sterilizing effect and on the other hand to eliminate the hydrogen peroxide. The package thus sterilized on the inside is then filled with sterilized product and closed in a sterile chamber.
Both these known methods for packaging sterilized products, e.g. sterilized milk, are used commercially, but it has proved a major difficulty and factor of uncertainty that the sterilization of the product and the sterilization of the packing material have to be carried out in two separate stages and product and package have to be combined thereafter. The uncertainty lies, among other things, in that the product has to pass a number of valves and pipelines of different types where a risk of infection is always present in pipe joints, valve seals etc.
It would be an advantage, therefore, if the heat treatment could be carried out in one stage in which the product and the packing material would be jointly sterilized and the product enclosed in the packing material. Such a process is known in itself, e.g. from Swedish patent specification No. 7307105-2 which describes an apparatus wherein a product, e.g. milk, is introduced into a plastic tube, this plastic tube being conducted through a heating zone, e.g. heated liquid, and is cooled down again thereafter. During the treatment in the hot bath the tube is compressed between rollers so that it is given a reduced area of flow which means that the liquid which is fed to the tube within the region which has a reduced area of flow will be at a higher speed in relation to the tube. This means that the liquid is given a shorter residence time in the heating zone than the tube of packing material, but owing to the tube being flattened, the column of liquid which runs through the tube will be very narrow so that all parts of the liquid come into good thermal contact with the heated walls of the tube. After heating and cooling, the tube is widened again so that the cross-sectional area increases and the relative speed between liquid and tube diminishes. At the actual packing, the speeds of the liquid and of the tube will be the same.
Such a known arrangement has the disadvantage, however, that it is difficult to carry on the heat treatment so long as the liquid which passes through the tube is heated to a temperature which exceeds its boiling point. A boiling of the liquid brings about a formation of steam which renders the process impossible and which prevents further heating. It is necessary therefore that the treatment should be done under pressure so that the liquid can be heated to approx. 140.degree. C. without boiling. To this end, a pressure of approx. 2.6-3 atmospheres above atmospheric pressure is required, that is, a pressure which the actual tube cannot withstand if the tube material consists of a thin plastic film, especially if the tube is heated to a temperature which exceeds the softening point of most plastic materials.
This technical problem has been solved, however, in accordance with the invention by a method which is characterized in that the product is introduced into a tube of flexible material, whereupon the tube containing the product is introduced between two substantially parallel bands movable synchronously in their longitudinal direction, and made preferably of heat-conducting material. The tube is received and compressed between the bands so that the tube is transported with the bands at the same time as the cross-sectional area of the tube is reduced and the product is made to flow forward through the compressed tube in a gaplike space of substantially uniform gap width. Heat is generated in, or is transferred to a zone of the movable bands which, through conduction and convection, transfer the thermal energy required for the treatment to the tube and the product contained in the tube. The tube and the product, after the heat treatment has been carried out during the necessary period, are cooled through thermal energy being given off to, or through another zone of the said bands.